bug#10754: define-minor-mode and doc of derived mode

[Eli Zaretskii]

> From: Stefan Monnier <monnier@IRO.UMontreal.CA>
> Cc: John Shahid <jvshahid@gmail.com>, 10754@debbugs.gnu.org,
>         drew.adams@oracle.com
> Date: Fri, 29 Jun 2018 09:55:22 -0400
> 
> > Sounds reasonable, I was hesitant to make the implementation more
> > complicated but it turns out not to be that bad. Attached the patch to
> > either append the paragraph or insert it at the first empty line.
> 
> Thanks, looks great to me.
> Eli, any objection?

No objections, but a few minor nits, after which this can go in.  The
master branch, I presume?